Method for making an HCL adsorbent

ABSTRACT

An HCl adsorbent, and method of making and using the adsorbent, the adsorbent comprising an activated alumina promoted with an alkali metal in an amount such that if calculated as alkali metal oxide, the adsorbent contains at least about 5% by weight alkali metal oxide.

This is a divisional of applications Ser. No. 08/176,415, filed on Dec.30, 1993, now U.S. Pat. No. 5,505,926, a divisional of Ser. No.07/878,857, filed on May 5, 1992, now U.S. Pat. No. 5,316,998, issuedMay 31, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to adsorbents for removing HCl from fluidstreams. More particularly, the present invention relates to a promotedalumina adsorbent for removing HCl from fluid streams.

2. Description of the Background

HCl is present as an impurity in numerous industrial fluids, i.e. liquidand gas streams. For example, in reforming processes conducted inrefineries, a chlorine promoted catalyst is generally employed. Duringthe reforming operation, which also generates hydrogen, small amounts ofgaseous HCl are produced which is carried away with the hydrogen. Thehydrogen containing the HCl is generally recycled to the reformer.Accordingly, the small amounts of gaseous HCl present in the recyclehydrogen can seriously interfere with the operation of the process and,in addition, can cause corrosion problems. Additionally, there are otherprocesses in which small amounts of HCl are generated and carried awayin gas or liquid streams and which must be removed from such streams toprevent corrosion damage to equipment and avoid environmental problems.

It is well known that activated alumina can act as a scavenger for theremoval of small quantities of HCl from fluid streams. For example, U.S.Pat. Nos. 4,639,259 and 4,762,537 both disclose alumina based adsorbentsfor removing HCl from gas streams. Typically, HCl scavengers made fromalumina are formed into nodules, e.g., spheres, the spheres forming afixed bed through which the gas to be scavenged flows. Handling and useof the nodules dictates that they possess sufficient crush strength toretain structural integrity. It is also known that aluminas impregnatedwith up to 5% sodium carbonate or sodium hydroxide exhibit improvedperformance in terms of enhanced hydrogen chloride absorption capacity.This is especially desirable since it means that the user can enjoylonger run times before having to change out the adsorbent, or in thecase of new units can design the units smaller. By increasing thecontent of promoters such as sodium carbonate, or sodium hydroxide, theHCl adsorbing capacity of the scavenger can be increased. However,heretofore, attempts to increase the content of such promoters hasresulted in a scavenger which does not possess sufficient structuralintegrity to be useful in commercial applications.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved scavenger for removing HCl from fluid streams.

Another object of the present invention is to provide an aluminaadsorbent for use as an HCl scavenger, the adsorbent having increasedHCl removal capacity.

Still a further object of the present invention is to provide a processfor removing HCl from fluid streams using alumina based adsorbents whichpossess excellent structural integrity and enhanced capacity for HClremoval.

Another object of the present invention is to provide a method formaking an alumina based adsorbent for use in the removal of HCl fromfluid streams.

The above and other objects of the present invention will becomeapparent from the drawings, the description given heroin and theappendent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a comparison of HCl adsorption capacity asbetween the adsorbent of the present invention and a prior art adsorbentcontaining sodium oxide derived from sodium hydroxide.

FIG. 2 is a graph similar to FIG. 1 comparing the adsorbent of thepresent invention with a prior art adsorbent made from alumina and a YZeolite.

FIG. 3 is a graph similar to FIG. 1 comparing the adsorbent of thepresent invention with several prior adsorbents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The primary component of the HCl adsorbent of the present invention isan activated alumina. The term "activated alumina" refers to an aluminahaving an extended surface area of above about 100 m² /g, preferablyabove about 150 m² /g. Methods for activation of alumina are well knownin the art. One technique which has been found to be particularly usefulis described in U.S. Pat. No. 2,915,365, incorporated herein byreferences for all purposes. In a common method of obtaining anactivated alumina, an alumina hydrate, e.g. bauxite, is heated at a hightemperature generally for a very short period of time in a process knownas flash calcination. Typically flash calcination or activation involvestemperatures of 400°-1000° C. with contact times of the order of 1 toseveral seconds, typically about 1 second. During this activation thealumina starting material is converted from a very low surface areahydrate to a high surface area material.

As a starting material to obtain the activated alumina, any number ofvarious aluminas or alumina containing materials can be employed. Forexample, essentially pure aluminas such as alumina trihydrate,pseudoboehmite, alpha alumina monohydrate can be used. A particularlyconvenient source of alumina starting material is gibbsite, a form ofalumina trihydrate, which is manufactured by the well known Bayerprocess. This product is readily available commercially and typicallyhas a particle size of 90-100 microns. In addition, the aluminacontaining material can comprise materials such as bauxite or, indeed,can be other alumina bearing sources such as beneficiated clays. Anotheruseful source of alumina containing materials are aluminas, e.g.boehmite, obtained from the hydrolysis of aluminum alkoxides. Ingeneral, the starting material alumina should have a minimum alumina(Al₂ O₃) content of at least about 40% by weight calculated on the basisof its dry weight, i.e., after ignition at 1000° C. for one hour. Thealumina used in the adsorbent must be reduced in size to the 1-10 micronrange, either before, or after being flash calcined but in any eventbefore being impregnated with the promoter.

As noted, the activated aluminas used in the present invention willgenerally possess a surface area in excess of 100 m² /g, preferably inexcess of 150 m² /g and more preferably in excess of 200 m² /g, and havea total pore volume in excess of 0.3 cc/g. The loss of ignition (LOI) ofthe activated alumina is typically 6-10% which contrasts to a loss ofignition of a starting material alumina, e.g., an alumina trihydrate,which is in excess of 30% weight.

In addition to an activated alumina, the HCl scavenger of the presentinvention comprises a promoter which is an alkali metal, believed to bein the scavenger in the form of an alkali metal oxide, but perhapspresent in the adsorbent (scavenger) in some other chemically boundmanner, e.g., a sodium aluminate. The alkali metal promoter used in theadsorbents of the present invention is derived from a water soluble,alkali metal salt of an organic compound which decomposes, i.e., theorganic portion of the compound is essentially destroyed or liberated,at temperatures below about 500° C. Thus, the sodium, potassium,lithium, etc., salts of such organic compounds can be employed, thesodium salts of such compounds being preferred. The organic portion ofthe alkali metal salts which serve as a source of the promoter used inthe adsorbents of the present invention can comprise any number oforganic moieties, the primary requisite being that the organic moiety isessentially destroyed or liberated from the compound (salt) attemperatures below about 500° C. Typical, non-limiting examples ofsuitable alkali metal salts which can be employed include carboxylicacid salts, especially the carboxylic acid salts of mono, di- andtribasic carboxylic acids containing from 1-6 carbon atoms. Especiallypreferred are the monocarboxylic acid salts of alkali metals wherein thecarboxylate portion of the salt contains from 1-6 carbon atoms. Suitablesuch salts include, without limitation, sodium or potassium formate,sodium or potassium acetate, sodium or potassium propionate, sodium orpotassium butyrate, etc. Especially preferred are the formates, acetatesand propionates of sodium.

The promoter may be incorporated into the alumina by several techniques.The activated alumina and the source of the alkali metal promoter, i.e.,the alkali metal salt described above, can be intimately mixed in powderor dry form, and the powder formed, in the presence of water, to thedesired shape. Alternately, a viscous slurry of the alumina containingmaterial and the source of the promoter can be extruded and pelletizedthereby forming an intimate mixture of the two primary components. In apreferred method of making the scavenger/adsorbent of the presentinvention, the alumina is formed into an appropriate shape oragglomerate, the source of the promoter being incorporated during theshaping process. For example, by using a rotating pan, well known tothose skilled in the art, and spraying in an aqueous solution of thealkali metal salt, nodules of adsorbent in the form of spheres can beformed. In this process using a pan nodulizer, the activated aluminapowder is fed into the rotating pan at a steady rate using a constantrate feeder. An aqueous solution of the alkaline metal salt is sprayedonto the alumina powder while it is in the constantly rotating pan. Thisprocess steadily turns the alumina powder into spheres which continuallygrow until they reach the desired size, usually from about 2 to about 4mm in diameter. While the preferred form of the scavenger are nodulessuch as spheres, it will be recognized that any shape can be employed.Thus, cylindrically shaped pellets, irregular lumps, or virtually anyother shape can be employed. Regardless of the method used toincorporate or impregnate the source of the alkali metal promoter intothe alumina, there must be an amount of the alkali metal salt presentwhich, when the adsorbent has been heat treated as described below, thealkali metal will be present in the adsorbent in an amount such that,calculated as alkali metal oxide, the alkali metal oxide content of theadsorbent will be at least about 5% by weight and preferably from about5-15% by weight based on the weight of alumina (Al₂ O₃).

In cases where the alumina containing material is a hydratable alumina,e.g bauxite, alumina trihydrate, etc., once the alumina has beenactivated, it is necessary that after it has been impregnated with thealkali metal salt and shaped into the appropriate form, it must be curedso that the alumina can be rehydrated. It will be understood, in thisregard, that there are commercially available aluminas which can beemployed as the alumina containing material and which, even afteractivation, do not require curing, i.e. rehydration. Rehydration ofhydratable aluminas is generally an exothermic reaction and usuallyrequires no additional heat. However, if ambient temperature conditionswarrant, heat can be added during the curing process so as to maintainthe curing temperature of the shaped adsorbent at above about 50° C. andpreferably above about 60° C. for a period of time sufficient torehydrate the alumina. The rehydration/curing process can be conductedis as little as several minutes if carded out under certain conditions.Alternately, the rehydration process can occur for extended periods oftime as for example up to 20 hours or longer. It will be understood thatin the curing process water must be present to provide the water ofhydration. In cases where the shaped adsorbent has been formed byspraying a water solution of the alkali metal salt onto the nodules asthey are formed, the requisite water is already present in the shapedmaterial. However, if required, the necessary amount of water forhydration may be supplied by the appropriate addition of water vaporusing above ambient pressures if necessary.

In the last step of the method of making the adsorbent, the shapedscavenger, cured if necessary, is heat treated at a temperature of fromabout 300° to about 555° C., more preferably from about 400° to about500° C., especially from about 425° to about 480° C. with a residencetime of from about 10-60 minutes, preferably from about 20-40 minutes.This heat treating can be accomplished in a standard type of activatorwell known to those skilled in the art. During this heat treating oractivation step, a major portion of the water of rehydration is removedfrom the scavenger. Additionally, to optimize the physical strength ofthe scavenger it is desirable, during the heat treating process, toreduce the LOI below about 5% by weight preferably in the range of fromabout 3% to about 5% by weight.

The adsorbents of the present invention, particularly when made using ahydratable alumina such as gibbsite, are characterized by a pore volumedistribution of at least 0.10 cc/g at a pore diameter of 750 Angstromsand greater, of at least 0.35 cc/g at a pore diameter of 40 Angstromsand greater and at least 0.16 cc/g at a pore diameters in the range offrom about 10,000 Angstroms to about 100 Angstroms. Additionally, theadsorbents have a crush strength of at least 15 lbs on a 5 mesh sphere.

The activated, promoted alumina adsorbent of the present invention canbe readily employed in the removal of HCl from industrial fluid, i.e.both gas and liquid, streams. Generally, in a typical process, theremoval of HCl is accomplished by providing a suitable adsorber vesselcharged with the adsorbent to form a fixed bed, and the HCl contaminatedfluid is conducted through the adsorber, either in a down or in anupflow manner. It has been observed that the best scavenging activitycan be achieved by, treating such streams containing up to about 1% byvolume HCl. Larger quantities of HCl in the streams may cause prematuresaturation of the scavenger with the possibility of having an undesiredearly break-through and consequent corrosion and environmental problems.It has been found that the promoted scavenger of the present inventionis effective in removing HCl from fluid streams when the level of HClranges from less than 1% by volume to less than 1 ppm by weight thusproviding an exit fluid of significantly reduced HCl content. Further,it has been found that, even if the HCl contamination of the fluidstream is in the range of from about 2 to about 4 ppm, the scavenger isstill capable of reducing the HCl concentration to levels below the 1ppm limit. The adsorbent of the present invention will also performeffectively in the presence of water, e.g. as vapor, in the gas beingpurified.

Purification of HCl contaminated fluid streams with the adsorbent of thepresent invention is generally continued until the fluid exiting fromthe purifying column is observed to have an HCl content above thedesired level. At this stage, the fluid to be purified is conducted intoa column filled with fresh promoted adsorbent and the used adsorbent iseither discarded or employed for other purposes.

To more fully illustrate the invention, the following non-limitingexamples are presented.

EXAMPLE 1

Gibbsite powder was flash calcined at 980° C. for 2 seconds to producean alumina powder (1-10 microns). The calcined alumina had a surfacearea of about 290 m² g. In making the adsorbents, the following generalprocedure was followed. The powdered, calcined alumina was fed to a pannodulizer and sprayed with an aqueous solution of the particular saltuntil the formed nodules reached the desired size, about 3 mm indiameter. The concentration of the aqueous solutions of the varioussalts were selected so as to achieve various levels of promoter in thefinal product. In all cases, the formed nodules impregnated with thepromoter were cured at 65° C. for 16 hours in an essentially sealedcontainer. Following this curing, the nodules were heated at 450° C. forabout 30 minutes.

The adsorbents thus prepared were tested for HCl adsorption, and dry andwet crush strength. For purposes of determining HCl adsorption a columnwas loaded with the promoted adsorbent through which a nitrogen streamcontaining 0.1% by volume HCl was passed continuously for 14 days at arate of about 0.2 liters/min. In each test, the column containedsubstantially the same amount of adsorbent.

In addition to conducting studies on the adsorbents prepared as above,HCl adsorption data and crush strength measurements were also obtainedon a calcium promoted adsorbent made in accordance with the teachings ofU.S. Pat. No. 4,639,259 and marketed by LaRoche under the name ofA-203Cl. The data is shown in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                                           Wet Crush                                               Wt. % Cl  Original Crush                                                                            Strenght                                   Adsorbent    Adsorbed  Strength in lbs                                                                           in lbs.                                    ______________________________________                                        Alumina + sodium                                                              carbonate.sup.1                                                               1. 4.0% Na.sub.2 O                                                                         13.2     31.8        18.0                                        2. 10.0% Na.sub.2 O                                                                        18.0     9.6         5.2                                         Alumina + sodium                                                              acetate.sup.2                                                                 3. 4.0% Na.sub.2 O                                                                         13.1     25.5        15.1                                        4. 10.0% Na.sub.2 O                                                                        17.7     26.6        14.7                                        5. A-203 Cl.sup.3                                                                          12.4     15.3        8.6                                         ______________________________________                                        Test Conditions:                                                                        0.1% HCl in nitrogen for 14 days for                                          adsorption data Wet crush measured after                                      exposure to 60% relative humidity for 24                                      hours. Original crush measured on dry sphere.                                 All crushes measured on 5 mesh sphere.                              ______________________________________                                         .sup.1 Prior art adsorbents made by spraying 8.0 wt. % solution of            Na.sub.2 CO.sub.3 (4.0% Na.sub.2 O) and 20.0 wt. % solution of Na.sub.2       CO.sub.3 (10.0% Na.sub.2 O). Pore volume distribution of 0.26 cc/g at a       pore diameter of 750 Angstroms or greater, 0.33 cc/g at a pore diameter o     40 Angstroms and greater, and 0.082 cc/g at pore diameters in the range o     10,000 Angstroms to 100 Angstroms.                                            .sup.2 Adsorbents according to present invention, made by spraying 12.0%      solution of sodium acetate (4.0% Na.sub.2 O) and 30.0 wt % solution of        sodium acetate (10.0% Na.sub.2 O). Pore volume distribution of 0.19 cc/g      at a pore diameter of 750 Angstroms and greater, 0.40 cc/g, at a pore         diameter of 40 Angstroms and greater, 0.16 cc/g at pore diameters in the      range of 10,000 Angstroms to 100 Angstroms.                                   .sup.3 Adsorbent as per U.S. Pat. No. 4,639,259 (Calcuim promoted)       

As can be seen from the data above, the prior art adsorbent made withsodium carbonate (No. 2 in Table 1), while showing adequate HCladsorption over the 14 day period, shows unacceptably low crush strengthat the 10% sodium oxide level. Indeed, the crush strength at the 10%sodium oxide level, both wet and dry, is so low as to render the productcommercially unusable in typical applications. This is to be contrastedwith the adsorbent of the present invention (No. 4 in Table 1) which atthe 10% sodium oxide level exhibits excellent crush strength and HCladsorption capacity. The adsorbent prepared in accordance with U.S. Pat.No. 4,639,259 (No. 5 in Table 1) shows markedly lower HCl adsorptionthan the adsorbent made according to the present invention.Additionally, it exhibits lower crush strength, both wet and dry, thanthe adsorbent of the present invention. Note that when the sodium oxidecontent is at 4% (No. 3 in Table 1), the adsorbent made by the processof the present invention does not show any better HCl adsorptioncapacity than what is exhibited by the prior art adsorbents.

EXAMPLE 2

Various of the adsorbents utilized in Example 1 were tested for HCladsorption over an extended 28 day trial. In all cases, the samequantity of adsorbent was employed and a gas stream containing 10 ppmHCl in air at a flow rate of 3.5 liters/min was used. The data are shownin Table 2 below.

                  TABLE 2                                                         ______________________________________                                        Wt. % of HCl adsorbed with time                                               Adsorbent    7 Days   14 Days  21 Days                                                                              28 Days                                 ______________________________________                                        Alumina + sodium                                                              acetate                                                                       3. 4.0% Na.sub.2 O                                                                         2.7      6.4      7.9    9.6                                     4. 10.0% Na.sub.2 O                                                                        3.6      7.6      11.0   13.0                                    5. A-203 Cl  3.1      5.6      7.5    9.4                                     ______________________________________                                        Test Conditions:                                                                        10 ppm HCl in nitrogen at 20.8 liters/min                                     for 28 days                                                         ______________________________________                                    

As can be seen from the data in Table 2, the adsorbent (No. 4) madeaccording to the process of the present invention, i.e. containing inexcess of 5% sodium oxide, shows markedly enhanced HCl adsorption ascompared with the prior art product (No. 5) produced according to U.S.Pat. No. 4,639,259 or with an adsorbent containing less than 5% sodiumoxide (No. 3).

EXAMPLE 3

Adsorbent (No. 4 from Table 1) was compared with a prior art adsorbentcontaining alumina and 4% by weight sodium oxide derived from sodiumhydroxide over a 15 day period for HCl (chloride) adsorption. The testconditions involved using air containing 100 ppm HCl at a flow rate of3.5 liters/min. The results are depicted in FIG. 1. As can be seen fromFIG. 1, over the 14 day test period, the adsorbent of the presentinvention (No. 4 from Table 1) shows a markedly greater HCl adsorptioncapacity as compared with the prior art adsorbent made from alumina andsodium hydroxide.

EXAMPLE 4

In this example, the adsorbent of the present invention (No. 4 fromTable 1) was compared with a prior art adsorbent made in accordance withthe teachings of U.S. Pat. No. 4,762,537, (the adsorbent containsalumina and Y zeolite) and marketed as Selexsorb HCl by Alcoa. HCladsorption tests on the two adsorbents were conducted as per theprocedure of Example 3 with the exception that an air stream containing1000 ppm HCL was employed at a flow rate of 3.5 liters/min. As can beseen from FIG. 2 the adsorbent of the present invention shows a markedlyenhanced ability to remove HCl as compared with the prior art adsorbentmade from alumina and Y zeolite.

EXAMPLE 5

Using the general procedure of Example 1, adsorbent No. 4 from Table 1,A-203 Cl (No. 5 from Table 1) and the prior art sodium hydroxide dopedalumina adsorbent from Example 3 were subjected to a protracted (˜50days) study to determine relative HCl adsorption capacity. The gas usedwas air containing 10 ppm HCl at a flow rate of 3.5 liters/min. Theresults are depicted in FIG. 3. As can be seen, over a long time periodthe adsorbent of the present invention is far superior to the prior artadsorbents using a calcium promoter (A-203 Cl) or promoted with sodiumwherein the sodium oxide content of the alumina is less than about 5% byweight and is derived from sodium hydroxide.

As can be seen from the data above, the present invention provides animproved HCl adsorbent with enhanced HCl adsorption capacity as comparedwith prior art adsorbents. Moreover, the adsorbent of the presentinvention exhibits acceptable crush strength which is important incommercial applications. This is particularly surprising since, asdemonstrated by the data above, prior art attempts to utilize anactivated alumina containing a high content, e.g. >5%, of an alkalimetal such as sodium results in an adsorbent which exhibits enhanced HCladsorption capacity (See Table 1, No. 2) but which has unacceptablecrush strength rendering it unfit for commercial applications. As can befurther seen from FIG. 3, by using the adsorbent of the presentinvention a user can obtain much longer run times.

While the invention has been demonstrated with respect to gas streams,it will be understood that it is equally applicable to liquid streamswhich will not deleteriously affect the adsorbent. For example, theadsorbent can be used to remove HCl from liquid hydrocarbons and otherorganic streams.

What is claimed is:
 1. A method for making an HCl scavenger compositioncomprising:impregnating an alumina containing material with a watersoluble, alkali metal salt of an organic compound which decomposes at atemperature below about 500° C. to form a promoted alumina; and heattreating said promoted alumina at a temperature of from about 300° toabout 550° C. for a time sufficient to remove a major portion ofhydrated water and provide an alumina adsorbent containing an alkalimetal present in an amount such that, if calculated as alkali metaloxide, said alumina adsorbent contains at least about 5% by weightalkali metal oxide based on the weight of alumina present in saidadsorbent, said adsorbent having a pore volume distribution of at least,0.10 cc/g at a pore diameter of 750 Angstroms and greater, of at least0.35 cc/g at a pore diameter of 40 Angstroms and greater, and at least0.16 cc/g at pore diameters in the range of from about 10,000 Angstromsto about 100 Angstroms, said adsorbent having a crush strength of atleast 15 lbs.
 2. The method of claim 1 wherein said alumina containingmaterial is an alumina hydrate.
 3. The method of claim 1 wherein saidalumina containing material is bauxite.
 4. The method of claim 1 whereinsaid alumina in said alumina containing material has a surface area inexcess of 100 m² /g.
 5. The method of claim 1 wherein said aluminacontaining material comprises a hydratable alumina and said impregnatedhydratable alumina is cured in the presence of water at a temperature ofabove about 50° C. for a period of time sufficient to hydrate saidhydratable alumina.
 6. The method of claim 5 wherein said hydratablealumina is formed into a plurality of adsorbent nodules, saidimpregnation with said alkali metal salt being conducted while saidnodules are being formed.
 7. The method of claim 6 wherein said nodulescomprise spheres.
 8. The method of claim 6 wherein an aqueous solutionof said alkali metal salt is used to impregnate said nodules.
 9. Themethod of claim 1 wherein said alkali metal salt is an alkali metal saltof an organic acid.
 10. The method of claim 9 wherein said organic acidhas from 1 to about 6 carbon atoms.
 11. The method of claim 10 whereinsaid alkali metal comprises sodium.
 12. The method of claim 11 whereinsaid organic acid comprises acetic acid.